cephfs: ideal PG ratio between metadata and data pools

Scenario: I'm deploying CephFS for the first time. I know I need a metadata pool as well as a data pool, but I don't know how many PGs for one pool and the other? Can I use the same number, or should they be different?

This question is a FAQ. The number of PGs should be chosen in line with the expected amount of data. In a typical cephfs usage pattern, there will be less data in the metadata pool than in the data pool, but nobody really knows exactly how *much* less. In this thread, John Spray suggests "something like" a 1:4 metadata-to-data ratio.

http://lists.ceph.com/pipermail/ceph-users-ceph.com/2017-March/016965.html

Quoting John's answer there:

As you suggest, it is probably sensible to use a smaller number of PGs
for the metadata pool than for the data pool.  I would be tempted to
try something like an 80:20 ratio perhaps, that's just off the top of
my head.

Remember that there is no special prioritisation for metadata traffic
over data traffic on the OSDs, so if you're mixing them together on
the same drives, then you may see MDS slowdown if your clients
saturate the system with data writes.  The alternative is to dedicate
some SSD OSDs for metadata.